Phong Nha Ke Bang
The chief reason to visit Dong Hoi is to take a day-trip or overnight trip to the Ke Bang National Park, where the main attraction is the spectacular Phong Nha cave. This is one of the world's great caves, and if you only visit one cave in Vietnam, make it this one.
To get to the cave, take Tran Hung Dao west, go right at the train station, and then take a left. The road joins up with Highway 15, which the locals call the Ho Chi Minh Highway (not to be confused with Highway 14 which is also called the Ho Chi Minh Highway). Highway 15 leads another 50 km or so west to the cave -- prominent signage will lead you through town, and the boat landing is at the end of the main road on the right. You can't reach the cave by road, you have to take a boat from the landing to the caves, 5 km down the river. Boats cost 150,000 VND and seat up to 14 people.
For more detailed information, check out section on Phong Nha town.